The present invention relates to an OIC/OS material, and especially relates to an OIC/OS material having a stable cubic crystal structure.
Solid electrolytes based on zirconia (ZrO2), thoria (ThO2) and ceria (CeO2) doped with lower valent ions have been extensively studied. The introduction of lower valent ions, such as rare earths (Y, La, Nd, Dy, etc.) and alkaline earths (Sr, Ca and Mg), results in the formation of oxygen vacancies in order to preserve electrical neutrality. The presence of the oxygen vacancies in turn gives rise to oxygen ionic conductivity at high temperatures (e.g. greater than 800xc2x0 C.). Typical commercial or potential applications for these solid electrolytes includes their use in solid oxide fuel cells (SOFC) for energy conversion, electrochemical oxygen sensors, oxygen ion pumps, structural ceramics of high toughness, heating elements, electrochemical reactors, steam electrolysis cells, electrochromic materials, magnetohydrodynamic (MHD) generators, hydrogen sensors, catalysts for methanol decomposition, potential hosts for immobilizing nuclear waste, and oxygen storage materials in three-way-conversion (TWC) catalysts.
Stabilized ZrO2 has been studied as the most popular solid electrolyte. In the case of doped ZrO2 both partially and fully stabilized ZrO2 have been used in electrolyte applications. Partially stabilized ZrO2 consists of tetragonal and cubic phases while the fully stabilized form exists in the cubic fluorite structure. Both CeO2 and ThO2 solid electrolytes exist in the cubic crystal structure in both doped and undoped forms. The amount of dopant required to fully stabilize the cubic structure for ZrO2 varies with dopant type. For Ca it is in the range of 12-13 mole %, for Y2O3 and Sc2O3 it is greater than 18 mole % Y or Sc and for other rare earths (Yb2O3, Dy2O3, Gd2O3, Nd2O3 and Sm2O3) it is in the range of 16-24 mole % of Yb, Dy, Gd, Nd, and Sm.
Fully or partially stabilized ZrO2, as well as other commonly studied solid electrolytes, have a number of drawbacks. In order to achieve sufficiently high conductivity and to minimize electrode polarization the operating temperatures have to be very high, in excess of 800-1,000xc2x0 C. For solid oxide fuel cells for example, reducing the operating temperatures below 800xc2x0 C. would result in numerous advantages such as greater flexibility in electrode selection, reduced maintenance costs, reduction in the heat insulating parts needed to maintain the higher temperatures and reductions in carbonaceous deposits (soot) that foul the operation of the fuel cell.
Further, in the automotive industry there is great interest in developing lower temperature and faster response oxygen sensors to control the air to fuel ratio (A/F) in the automotive exhaust. In the case of three-way-conversion (TWC) catalysts solid solutions containing both ZrO2 and CeO2 are used as oxygen storage (OS) materials and are found to be more effective than pure CeO2 both for higher oxygen storage capacity and in having faster response characteristics to A/F transients.
Oxygen storage capacities (OSC) in these applications arises due to the facile nature of Ce4+ less than xe2x86x92 greater than Ce3+ oxidation-reduction in typical exhaust gas mixtures. The reduction of the CeO2 to Ce2O3 provides extra oxygen for the oxidation of hydrocarbons (HCs) and CO under fuel rich conditions when not enough oxygen is available in the exhaust gas for complete conversion to carbon dioxide (CO2) and water (H2O). The use of binary CeO2/ZrO2 and ternary CeO2/ZrO2/M2O3 based materials in such applications have advantages over the use of pure CeO2 containing catalysts. This arises because in pure CeO2 only surface Ce4+ ions can be reduced in the exhaust at typical catalyst operating temperatures of 300-600xc2x0 C. (See FIG. 1). However, in binary CeO2/ZrO2 or ternary CeO2/ZrO2/MxOy solid solutions more oxygen is made available through the reduction of bulk Ce4+ and the subsequent migration of xe2x80x98Oxe2x80x99 to the surface of the solid solution crystallites where it reacts with the HCs and CO as is demonstrated in FIG. 2.
The xe2x80x98Oxe2x80x99 migration to the surface of the solid solution crystallites is made possible by the formation of the solid solution and is thus an analogous process to that occurring when these same materials are used as solid solution electrolytes. Thus, a more accurate description of these materials for TWC catalyst applications is to view them as oxygen ion conducting/oxygen storage (OIC/OS) materials. These materials have a much higher oxygen storage capacity compared to pure CeO2, especially after catalyst aging and the formation of large crystallites. Further, the response of these solid solutions to changes in the exhaust gas environment is more rapid compared to pure CeO2 with the result that they operate more effectively in preventing CO/HC/NOx breakthrough during accelerations and they further provide oxygen at lower temperatures.
Aging of electrolytes is a phenomena usually associated with a decrease in the ionic conductivity at a constant temperature with time. The aging process is a function of composition, operating temperature, time and temperature cycling. The two main causes of aging are: a) ordering of the cation and anion sublattice and b) decomposition of the metastable phases. In single phase cubic systems the major cause of aging is formation and growth of microdomains and disproportionation at high temperatures into different phases. Aging of cubic Y stabilized ZrO2 oxygen ion conducting electrolytes for example can occur through disproportionation into a Y-rich cubic phase and a Y-poor tetragonal phase. Thus, phase stability at high temperatures is an important property of solid solution electrolytes and maintaining phase stability in an optimized cubic or tetragonal phase after high temperature operation or cycling is a highly desirable property.
For TWC catalyst applications the newest OIC/OS materials consist of a range of CeO2/ZrO2 solid solutions with lower valent dopants added to increase the number of oxygen vacancies and to increase the thermal stability and oxygen ion conductivity of the solid solutions after sintering at high temperatures. Zr-rich compositions have the advantage in that the reduction energies for Ce4+xe2x86x92Ce3+ decrease with increasing Zr content and that the activation energies for mobility of xe2x80x98Oxe2x80x99 within the lattice decreases. This is demonstrated in FIGS. 3 and 4 (Balducci et al., J. Phys. Chem. B., Vol. 101, No 10, p. 1750, 1997). (Line A is isolated Ce3+ and V0xe2x80x3 vacancies; B is Ce3+ minus V0xe2x80x3 clusters; and C is Ce3+ minus V0xe2x80x3 minus Ce3+ clusters.) However, the Zr-rich systems suffer from the disadvantage in that the oxygen storage capacity is decreased due to the lower CeO2 content. Thus, strategies to optimize the availability (OIC) of the OSC function go counter to those that maximize oxygen storage capacity (OSC).
A further disadvantage of the Zr-rich systems is that the stable crystal structure is tetragonal rather than the more desirable cubic structure. The crossover composition between cubic and tetragonal occurs in the range of 35-45 Mole % ZrO2. Compositions having higher ZrO2 content have the tetragonal crystal structure while compositions of lower Zr content are cubic. It has been found that compositions with the cubic crystal structure have more facile redox properties and respond faster to changes in A/F composition. The preferred cubic phase can be fully stabilized by inclusion of Y or other rare earths such as La and Pr in the ZrCeO2 crystal structure.
Loss of the OIC/OS properties as a function of aging for solid solutions used for TWC applications occurs via a number of mechanisms.
These include: a) decomposition of the meta stable phases; b) overall particle or crystallite growth and c) segregation of the OIC/OS function from the precious metals (PMs).
Decomposition of the meta stable phases can cause loss of OIC/OS properties. Essentially, aging of solid solutions with either cubic or tetragonal stabilized structures can result in disproportionation into a composite consisting of more Ce-rich cubic phases and more Zr-rich tetragonal phases. This is true for both intermediate Zr/Ce-content compositions and for the Ce-rich and Zr-rich parent phases. The degree of disproportionation is dependent on both temperature and exhaust gas environment. Typically, aging under reducing conditions results in much greater disproportionation, probably as a result of reduction of Ce4+ to Ce3+ which results in an expansion of the Ce ionic radius from 0.92 to 1.034 xc3x85 with a consequence being destabilization of the crystal structure. For TWC applications the most ideal crystal structure consists of the cubic phase, even for Zr-rich compositions. This phase can be stabilized by the incorporation of rare earths such as Y and to a lesser extent La and Pr into the crystal structure. Larger doping levels of La and Pr are required for full stabilization of the cubic structure as compared to Y.
Similarly, growth of particle or solid solution crystallites results in loss of oxygen storage capacity as Ce ions located in the interior of the large crystallites become inaccessible for redox activity. Further, with increasing Ce-content the fraction of Ce accessibility for reduction decreases for Ce contents above about 20 mole % Ce in the solid solution. This is demonstrated in FIG. 5 from a temperature programmed reduction (TPR) analysis of CeO2/ZrO2 compositions of varying Ce/Zr content, theoretical (dashed line), and those that were aged at 1150xc2x0 C. for 36 hours (solid line). The TPR experiment measures the rate of H2 uptake as a function of temperature in a 5% H2/95% Ar mixture. The lower the temperature for H2 consumption (Ce4+ reduction) the more facile the oxygen storage function. The oxygen storage capacity can be calculated from the total H2 uptake based on the reaction stoichiometry:
2Ce4+O2+H2xe2x88x92xe2x86x92Ce23+O3+H2O.
It is seen that for low Ce-content mixtures of up to 20 mole %, all the Ce is accessible, i.e. 100% of the Ce present in the solid solution is reduced. However, above 20 mole % addition of more Ce does not result in increased H2 uptake indicating that at the higher Ce-contents the added Ce is not redox active. Thus, improvements in the nature of the OIC/OS material that would increase the fraction of Ce reduction for Ce contents higher than 20 mole % are highly desirable.
Finally, segregation of the precious metals from the OIC/OS materials can cause a loss of OIC/OS properties. A loss of the OIC/OS function occurs as the precious metal and OIC/OS functions in fresh, non-aged catalysts couple to give rise to a synergistic reduction of both the Ce4+ and precious metal (c.f. B. Harrison; A. F. Diwell and C. Hallet., Platinum Metals Rev. Vol. 32, P. 73 (1988); H. C. Yao and Y. F. Yu Yao, J. Catal., Vol. 86, P. 254, (1984); J. G. Nunan, H. J. Robota, M. J. Cohn and S. A. Bradley; J. Catal., Vol. 133, P. 309 (1992); J. C. Nunan, SAE Paper 970467, Detroit, (1997)). when the precious metal and the Ce-containing solid solutions are in contact, reduction of both the Ce and precious metal occurs at lower temperatures for both than would happen if both were separate. The catalytic reduction of Ce4+ to the Ce3+ state by the precious metal is an important and extensively studied aspect of TWC operation. It has been shown that loss of this feature due to sintering and segregation of the precious metal-OIC/OS function results in loss of catalytic activity (c.f. S. H. Oh and C. C. Eickel, J. Catal., Vol. 112, P. 543, (1988); A. S. Sass, V. A. Shvets, G. A. Savel""era, N. M. Povova, and V. B. Kazanskii, Kinet. Katal., Vol. 27, P. 894, (1986); J. G. Nunan, W. B. Williamson and H. J. Robota, SAE Paper 960798, Detroit, (1996); J. G. Nunan, SAE Paper 970467, Detroit, (1997)). For this synergistic reduction feature to operate, both the precious metal and solid solutions have to be in direct contact. Loss of contact between the two components arises due to sintering of the both the precious metals and OIC/OS components. Thus, discovering a method to preserve this synergistic reduction feature after aging is highly desirable.
One way to prevent loss of this synergistic reduction feature would be to incorporate the PM directly into the solid solution matrix or crystal structure. However, this would be extremely costly and potentially wasteful as the PM serves other functions in the TWC catalysts not related to its promrotion of the OIC/OS function.
An alternative approach to prevent loss of the reduction feature would be to introduce a cheaper redox active element into the solid solution matrix that would serve the same function as the PM in promoting Ce4+ reduction. Such elements potentially include Fe, Ni, Co, Cu, Ag , Mn, Bi and mixtures of these elements. However, some of these elements are currently used in TWC catalysts for other purposes. Fe, Ni and Mn are used in commercial TWC catalysts for H2S control and are often impregnated with the oxygen storage function. This is because Ce is the primary source of H2S emissions (c.f. M. G. Henk, J. J. White and G. W. Denison, SAE paper 872134, Toronto, November, (1987); R. P. Cohn, J. M. Longo, U.S. Pat. No. 4,346,063; G. J. Barnes, J. C. Summers, SAE Paper 750093 (1975); J. S. Rieck, European Patent 0-385-123-A2(1990); G. Blanchard, E. Quemere; O. Touret, V. Visciglio, Eoropean Patent WO 96/21506, (1996)). H2S emissions arise as during lean (oxidizing conditions) operation the catalyst stores sulfur by formation of Ce, Al and other base metal sulfates. During rich transients (e.g. during an acceleration), these sulfates decompose and the released SO2 is rapidly reduced to H2S (i.e., the rotten egg odor associated with emission control catalysts) by the precious metals. The H2S is captured by the Ni, Fe and Mn oxides before it leaves the catalyst and is stored as a sulfide. Regeneration occurs during lean conditions when the sulfide decomposes and the sulfur is released as SO2 and exits the catalyst. It is important to note that in the application of base metal oxides for H2S control the base metal must exist as a discrete oxide and possess the chemical properties of the oxides for effective operation. Thus, mixtures of the form (Ce0.5Zr0.5O2)+(Fe2O3) could be used as distinct from a true solid solution which have the chemical formula of the type CeaZrbFecOz. A summary of the reactions is shown below for Ni:
Rich: H2S+NiOxe2x86x92NiS+H2O
Lean: NiS+1.5O2xe2x86x92NiO+SO2
In contrast to Ni, Fe, and Mn, Ag is used in TWC catalysts for more effective combustion of aldhehydes for fuels that contain alcohols as octane boosters.
What is needed in the art are OIC/OS materials having stable cubic crystal structures, facile and high oxygen storage and oxygen ion conductivity properties for high Ce-content solid solutions.
The present invention comprises an OIC/OS material, a catalyst comprising the OIC/OS material, and a method for converting hydrocarbons, carbon monoxide and nitrogen oxides using the catalyst. This OIC/OS material comprises: up to about 95 mole % zirconium, up to about 50 mole % cerium, up to about 20 mole % of a metal selected from the group consisting of yttrium, rare earths (ideally La, Pr or La+Pr) and a mixture of yttrium and a rare earth, and about 0.05 to about 25 mole % of a base metal selected from the group consisting of iron, copper, cobalt, nickel, silver, manganese, bismuth and mixtures comprising at least one of the foregoing metals.
The invention further comprises the reaction product of up to about 95 mole % zirconium, up to about 40 mole % cerium, up to about 15 mole % of a metal selected from the group consisting of yttrium, a rare earth and a mixture of yttrium and a rare earth, and about 0.05 to about 20 mole % of a base metal selected from the group consisting of iron, copper, cobalt, nickel, silver, manganese, bismuth and mixtures comprising at least one of the foregoing metals.
The catalyst comprises the OIC/OS material; a noble metal component; and a high surface area porous support wherein the zirconium, cerium, yttrium, a rare earth (lanthanum or praseodymium), noble metal and porous support are deposited on a substrate. Meanwhile, the method for converting hydrocarbons, carbon monoxide and nitrogen oxides in an exhaust stream, comprising: using the catalyst; exposing the exhaust stream to the catalyst; and converting hydrocarbons, carbon monoxide and nitrogen oxides in the exhaust stream to water, carbon dioxide and nitrogen.
The above described and other features of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.